1. Field of the Invention
The present invention relates to a mold cased circuit breaker which protects a circuit and load side equipment by automatically breaking the circuit when an overload or short-circuit occurs, and more particularly, to a contactor assembly for a circuit breaker.
2. Description of the Conventional Art
A mold cased circuit breaker (so called abbreviated MCCB) is installed at an electric power distributing board among several electric power systems of a factory and a building. The circuit breaker serves as a switch for supplying or cutting off power to a load side under no load state, and cuts off power supplied to a load side from a power source side in order to protect a circuit and components of the load side when a great abnormal current due to a short-circuit or a ground fault flows on a circuit under a load state.
FIG. 1 is a cross sectional view showing the main internal configuration of a conventional circuit breaker. FIG. 2 shows a movable contactor assembly for the conventional circuit breaker, which is a front view showing a status where contacts contact each other (On Status). FIG. 3 shows the movable contactor assembly for the conventional circuit breaker, which is a front view showing a status where the contacts are separated (Off Status).
As illustrated therein, the conventional circuit breaker 1 comprises a mold case 10, stationary contactors 9 and 9′ installed at regular intervals within the mold case 10 in horizontal and vertical directions, a rotary type movable contactor assembly 17 rotatably installed between the stationary contactors 9 and 9′, a trip mechanism 11 for sensing a large current and tripping the breaker, a switching mechanism 12 being operated automatically by the trip mechanism 11 or operated by manual manipulation of a handle 1a and for separating a movable contactor 4 from the stationary contactors 9 and 9′ and breaking the circuit, and an arc extinguishing mechanism 13 for extinguishing a high temperature, high pressure arc gas produced between a contact 4b of the movable contactor 4 and contacts 9b and 9b′ of the stationary contactors 9 and 9′ upon the on-off operation of the circuit.
The mold case 10 houses the above mechanisms, is formed of insulating material and serves to insulate between the mechanisms, between phases and from the outside and prevents this material from penetrating into the mold case 10.
The contacts 9a and 9b are formed at the ends of the stationary contactors, i.e., the stationary contactor 9′ at the power source side and the stationary contactor 9′ at the load side, and the contact 4b is formed at the end of the movable contactor 4.
The conventional movable contactor assembly 17 comprises a rotary shaft 2 being rotatably installed between the stationary contactors 9 and 9′ for maintaining an On status or Off status and being installed between the movable contactor 4 having cam surfaces 4c and cam grooves 4d and the stationary contactors 9 and 9′ for supporting the movable contactor 4, limit pins 19 being contacted with the cam surfaces 4c and the cam grooves 4d, and a plurality of springs 15 and 16 for elastically rotating the movable contactor 4 around a virtual rotational axis (rotation center) 2a by an electromagnetic repulsive force generated at each of the contacts 9b and 9b′ when a large current flows due to the abnormality in the circuit.
One ends 15a and 16a of the plurality of springs 15 and 16 are fixed to the limit pins 19, and the other ends 15b and 16b are fixed to the rotary shaft 2. For reference, a method of supporting the movable contactor 4 on the rotary shaft 2 around a virtual rotational axis (rotary center) 2a, though not actually existing, is referred to as self-centering.
As illustrated in FIG. 2, a status where the contact 4b of the movable contactor 4 and the contacts 9b and 9b′ of the stationary contactors 9 and 9′ are contacted with each other is referred to as “Off Status”, and as illustrated in FIG. 3, a status where the contact 4b of the movable contactor 4 and the contacts 9b and 9b′ of the stationary contactors 9 and 9′ are separated from each other is referred to as “On Status”. A status where the Off Status is being converted into the On status is referred to as separating and opening.
As illustrated in FIG. 2, in the Off status, a normal current flows through the circuit, whereupon the limit pins 19 presses the cam surfaces 4c of the movable contactor 4 by a constant pressure by means of the tensile strength of the springs 15 and 16, thereby maintaining the movable contactor 4 and the stationary contactors 9 and 9′ to be contacted with each other.
In contrast, as illustrated in FIG. 3, in the event that a large current flows through the circuit due to a short-circuit of the circuit, the movable contactor 4 rotates clockwise by an electromagnetic repulsive force between the contact 4b of the movable contactor 4 and the contacts 9b and 9b′ of the stationary contactors 9 and 9′. At this point, the contact 4b is separated from the contacts 9b and 9b′, thereby breaking the circuit. Unexplained reference numeral 18 represents a stopper for limiting the rotating range of the movable contactor.
However, in the movable contactor assembly for the conventional circuit breaker, the contact pressure of the springs 15 and 16′ is not constant, including the problems that contact pressure of the springs 15 and 16′ further increases when an over travel occurs by which the rotary shaft 2 rotates further than the movable contactor 4 at a contact position where the movable contactor 4 is stopped in contact with the stationary contactors 9 and 9′, and the contact pressure of the springs 15 and 16 changes according to the position of the limit pins on the cam surfaces 4c and cam grooves 4d. Therefore, when the movable contactor 4 rotates separately from the stationary contactors 9 and 9′ due to an electromagnetic repulsive force caused by the application of a large current, if the electromagnetic repulsive force becomes smaller than the contact pressure, the movable contactor 4 returns to the position contacting the stationary contactors 9 and 9′. As a result, the current limiting operation is failed. Due to this, an arc is continuously generated, and thus the instantaneous current limiting characteristic cannot be obtained, and a serious damage can be given to the circuit breaker and the load equipment.